1. Field of the Invention
This invention relates to a muddy water treating method wherein coagulating agents are added to muddy water to be treated to coagulate fine particles in muddy water into large-sized solid flocs and more particularly, to a treating method suitably used for the process of muddy water produced by the execution of a tunnelling operation, a boring operation, a pile driving operation, an operation of removing sedimentary earth in a storage lake and sewage treatment or the like.
2. Description of the Prior Art
A known method of removing fine particles from muddy water comprises adding one or more coagulating agents in muddy water in a treating vessel to combine fine particles in the muddy water into flocs and sedimentating the flocs by gravity. In this known method, muddy water is separated into a floc phase and an aqueous phase since the flocs are settled. The thus separated flocs are removed from the treating vessel and dewatered, i.e., dehydrated, and then wasted in position. In contrast, the separated water is finally returned to rivers.
In the known method, the coagulating agents are merely added, so that large-sized or hard flocs cannot be produced. In most case, fine particles are partially left in the liquid with the great possibility that the separated water is not transparent.
If the formed flocs are small in size, they are liable to run away along with the liquid at the time of dewatering or transport of the flocs, thus leading to a troublesome after-treatment of the flocs. Alternatively, if the flocs are soft with their mechanical strength being small, the flocs are apt to break. Especially, at the time of dewatering and transport, the flocs will be broken into fine particles, so that the broken flocs are run away along with water. The opaqueness of the separated liquid means that fine particles are left in the liquid with incomplete removal of the fine particles.
Accordingly, the coagulating agents have been conventionally added in large amounts, but the use of the large amounts of expensive chemicals will require much treating costs. Even if coagulating agents are used in large amounts under conditions where the concentration of fine particles in muddy water is high, either large-sized or hard flocs are not produced or the resultant water becomes opaque.
To overcome the above disadvantages involved in high muddy water concentration, it has been usual to add clear water to muddy water so as to lower the concentration, after which coagulating agents are added. However, the addition of clear water to the muddy water undesirably increase the amount of the total water to be treated.